An electric motor has been widely used in various fields such as industry and agriculture. In practical applications, the electric motor may experience over temperature due to overload, winding faults or abnormal power voltage so as to damage or destroy the electric motor, and thus the over temperature protection for the electric motor is needed. In addition, the electric motor should be safely stopped when it is in abnormal operation status. That is, a STO (safe torque off) should be applied manually to make the electric motor stop work.
In order to stop the electric motor when it is over temperature or a safe torque off signal is received, the existing control and protection apparatus is usually provided with a control unit comprising a thermistor, an OR gate circuit and a safety circuit, wherein the two input ends of the OR gate circuit are configured to receive the electric motor's temperature signal detected by the thermistor and the safe torque off signal (or safe stop signal) respectively. When the temperature of the electric motor is too high and/or the safe torque off signal applied manually is received, the safety circuit makes the inverter having an insulated gate bipolar transistor (IGBT) stop work, and thus makes the electric motor stop work. In the control and protection apparatus above, the OR gate circuit and the safety circuit need ATEX certification, which may increase the cost of the apparatus. On the other hand, upon a failure of the OR gate circuit, the applied safe torque off signal and/or over temperature of the electric motor will not be able to stop the electric motor, which means the performance of the OR gate circuit must meet higher requirements.